1. Field of the Invention
This invention relates to a new and improved capacitor array and, in particular, to a new and improved capacitor array for a companded pulse-code modulation voice codec. Accordingly, it is a general object of this invention is provide a new and improved array of such character.
2. Description of the Prior Art
Voice signals can be coded in a non-linear digital code for digital telephone transmission in order to maintain a high ratio of signal-to-noise over a wide signal level range without requiring excessive digital transmission bandwidth. One widely used code in the United States is the 15 segment approximation of the .mu.=255 compression law. An analog signal, such as speech, can be converted into a digital coded output using an 8-bit pulse-code modulation format in which, in each of the 15 segments, there are 16 equal steps (with the exception of the segment passing through the origin which contains 31 equal steps). In such case, a standard sampling rate for voice signals is 8 kilohertz, resulting in a 64 kilobits PCM signal.
Cameron, in U.S. Pat. No. 4,195,282, entitled "Charge Redistribution Circuits", utilizes a binary weighted capacitor array wherein the various capacitors are related to each other by the ratios of 1, 2, 4, 8, 16, 32, 64, and 128.
With such an exponential relationship, problems tend to occur with edge effects in the production of integrated circuits in that a 32 or 64 unit capacitor might not be thirty-two or sixty-four times as large as a smaller one, because of that edge effect. If one were to actually construct, for example, a 16 size capacitor by joining together a group of sixteen one unit capacitors, the fabrication required would be undesirably complicated. A limit to such a scheme would be to what extent a significant ratio could actually be fabricated on a practical basis.